1. Field of the Invention
The invention relates generally to antennas and in particular relates to horn antennas utilizing circularly polarized signals.
2. Prior Art
Horn antennas are generally known in the prior art, so too, are circular polarizers and cross-polarization attenuators. These microwave components have, until now, been separate entities which are serially connected together. The horn antennas which are used in satellite communications applications, for example, may be conical, square or have other equal multi-sided configurations. Heretofore, horn antennas merely provided the function of radiating or receiving the circularly polarized energy.
The circular polarizer section, which is usually mounted immediately adjacent to the horn antenna, only provides a rotation or circular polarization to a linearly polarized wave which is to be transmitted. The polarizers have generally consisted of a quarter-wave plate or 90 degree phase shifter placed in a cylindrical or square waveguide section. The quarter-wave plate may be made of a dielectric or conductive material. Another method of providing circular polarization is by utilizing fins inside a cylindrical or square waveguide section.
Attenuators are also generally known in the prior art for reducing the amplitude of cross-polarized waves in an antenna. The attenuators are usually connected between the polarizer section and a diplexing network for transmitting and receiving the microwave energy. The prior art attenuators include a waveguide section having a wedge-shaped resistive member mounted therein. The apex of the wedge is directed at the aperture of the horn antenna, i.e., the direction from which the unwanted energy is coming, and the plane of the wedge is parallel to the E vector of the cross-polarized linear wave. The wedge thusly oriented is transparent to a perpendicular input wave but is resistive to a parallel cross-polarized wave thereby attenuating the cross-polarized signals. Other methods of reducing the cross component of a linear signal include use of the magic "tee" or hybrid circuitry.
Another method of producing circularly polarized signals from a linear wave is to place an external screen or grating directly in front of the horn aperture which is radiating linear signals. The screen or grating is composed of a series of conductive strips arranged at a 45 degree angle to the direction of linear waves. The strips so arranged provide both right and left hand circular polarization to two orthogonal signals being radiated by the horn antenna. With such an arrangement an attenuator cannot be used because the radiated or received signals at the antenna will be linear and an attenuator as described above would completely eliminate one of the signals.
One of the principal drawbacks of having a system as above described i.e., a separate horn antenna, a separate circular polarizer and a separate attenuator is quite obviously the length and weight of such a combination. The weight and length of such prior art systems make them impractical for satellite communications applications. For example, a horn antenna used in a communications satellite broadcasts and receives 3.7 to 4.2 GHz. is approximately 4" in length. The polarizer section is approximately 8" in length and the attentuator section is 10" long. The transition waveguide section from the transmitter receiver to the input of the attenuator section is approximately 3" long. Thus, the entire antenna group is about 25" long and weighs approximately one pound. It is apparent that the use of separate microwave antenna components requires volume and adds greatly undesired weight to a communications satellite which is being placed into orbit around the earth.